If you're already lifting, then you need to EAT. A LOT. Seriously. If you are not getting enough calories to accommodate muscle gain, you'll stay the skiny guy. Get lots of protein, lots of good carbs and some unsat. fats. Try to eat 6x a day if you can, keep the nutrition coming. If you just don't have the appetite for it, get whey or weight gainer shakes. They're not so filling and can quickly up your overall calories. List what your current diet looks like (roughly/exactly what you eat, how much and when). I'm sure we can find some ways to bulk you up.